1. Field of the Invention
The present invention relates generally to a processing vessel and a method of mixing powders with a magnetically coupled agitator mounted inside of the processing vessel. More particularly the invention is directed toward a rotating processing vessel and a method of using a magnetically coupled agitator to mix dry or moist powders inside of the rotating powder processing vessel. Although the method of the present invention has many different applications, it is described herein primarily as used for mixing dry powders inside of a tumble blender.
2. Description of the Related Art
Rotating tumble blenders are frequently used to mix dry chemical compounds and other ingredients for the pharmaceutical, food, cosmetic and other industries. Because of the nature of these highly-regulated industries, a sterile mixing environment or an environment which is free from cross-contamination is oftentimes required during the mixing process. Besides operating under ambient pressure conditions, for some applications, the blender operates under pressures greater than atmospheric. For other applications, the blender operates under less than atmospheric pressure. To satisfy all of these applications, tumble blenders must be constructed and operated to both prevent contaminants from entering the vessel, such as when the vessel is operating at less than atmospheric pressure during a mixing process, as well as prevent the vessel""s contents from escaping to the environment outside the vessel walls such as when the vessel is operated under conditions greater than atmospheric pressure.
In some applications a tumble blender is outfitted with a rotating agitator designed to enhance and/or accelerate the mixing of the contents of the blender. In these instances the tumble blender typically includes a drive shaft that couples a mixing agitator (impeller) located within the interior of the vessel to a motor located outside of the blender vessel. To effect such an arrangement where the drive shaft penetrates the vessel wall, the vessel oftentimes contains mechanical seals and/or a packing arrangement located between the drive shaft and the vessel wall. These seals and packing are designed to prevent the vessel contents from migrating along the drive shaft into the bearings and ultimately out of the vessel particularly when the interior of the vessel is operating under greater than ambient pressure conditions. Additionally, the seals and packing prevent any outside contaminants from entering the vessel along the same route particularly when the interior of the vessel is operating under negative pressure conditions (less than atmospheric pressure). Such seals and packing arrangements are undesirable for the mixing applications referenced above because they are susceptible to failure, especially under pressurized or negative pressure conditions. Additionally, such seals, packing and bearing failures when the powders migrate past the seals add to the blender""s maintenance costs because they are difficult to clean and replace. Cleaning and replacement is required to prevent product which becomes entrained in the seals and packing during one mixing application from cross-contaminating a second, different, product during a subsequent mixing application within the same vessel.
In a typical dry mixing application, when an agitator rotates inside of the mixing vessel, work energy is added to the dry powders situated therein, creating heat. In some instances the heat energy increases the temperature of the air inside of the vessel causing a buildup of pressure. If there is no mechanism, such as an atmospheric vent or pressure relief valve, to allow the pressure to dissipate from within the vessel to the surrounding atmosphere, internal pressure within the vessel forces the powders back along the agitator shaft inside of the vessel. This causes the bearings, mechanical seals and/or packings to fail prematurely resulting in the escape of powders from the vessel to the outside environment.
In those instances where the vessel is vented to the atmosphere to alleviate any pressure build-up, filters or filter cloths are typically placed over the vents in an attempt to prevent the powders inside of the vessel from escaping. Inevitably, however, some powders do pass through these filter arrangements thus creating an environment detrimental to worker health and safety particularly where the powders are toxic or reactive. Furthermore, after a period of use the filters often become blinded by the powder. This blinding effectively seals the vent and allows pressure to build-up within the vessel. This increased pressure causes the mechanical seals and/or packings to fail prematurely. The use of pressure relief valves in these applications presents similar difficulties as they too often become clogged by the powder.
To eliminate these problems when mixing dry or moist powders in a rotating tumble blender, the processing vessel and method of the present invention uses a magnetic coupler to couple the mixing agitator on the interior of the blender to a motor located outside of the blending vessel. The magnetic coupler comprises on the outside of the vessel walls a magnet (the xe2x80x9cdrivexe2x80x9d magnet) attached to a shaft which is rotated by a motor and, on the interior of the vessel, another magnet (the xe2x80x9cdrivenxe2x80x9d magnet) connected to the agitator. In this manner the drive shaft does not penetrate the vessel walls and the need for seals and/or packing in the vessel walls is eliminated. The drive and driven magnets are assembled close together, although they are on opposite sides of the blender vessel wall, so that the rotation of the drive magnet rotates the driven magnet and hence the agitator. This magnetic coupling arrangement advantageously allows the mixing agitator inside the blender to be rotated by a motor outside of the blender without mechanically connecting the two members. Therefore, in the processing vessel and method of the present invention, pressurized or negative pressure conditions inside of the mixing vessel no longer present a problem for maintaining a sterile mixing environment since the drive shaft does not penetrate the vessel""s walls. Since the mechanical seals and/or packing associated with a traditional agitator application are eliminated, a conduit for cross-contamination between the interior and exterior of the vessel along the drive shaft is eliminated. Additionally, the need for an atmospheric vent and any associated filter or filter cloth is also eliminated.
Magnetic couplings in general are well known in the prior art for mixing or pumping liquids. Typically, such magnetic couplings comprise a pair of axially or radially opposed magnets, or sets of magnets, formed from a magnetic material. One of the magnets is coupled to a driving member such as a shaft from a motor, and the other magnet is coupled to a driven member such as a pump impeller or agitator. The magnets are magnetically coupled to each other so that rotation of the driving member causes a corresponding rotation of the driven member to obtain the desired torque output. Couplings of this type are particularly advantageous, as described above, when it is desirable for an impermeable barrier to be interposed between the driving and driven members such as in stirred reactors, autoclaves, centrifugal pumps and the like. In such applications, the barrier assures against passage or leakage of any process fluid being mixed between the driving and driven members, and thereby prolongs the operating life of the equipment. For examples of prior art mixers, stirrers and pumps employing magnetic couplings in liquid applications see U.S. Pat. Nos. 2,495,895; 2,556,854; 2,711,306; 2,996,393; 4,207,485; 4,247,792; 4,277,707; 4,534,656; 5,292,284; 5,407,272; 5,470,152 and 5,533,803.
During operation, a magnetic coupling may generate substantial quantities of heat due to relative slippage of the magnets at excessive torque loads, induction heating effects, and the like. This is particularly true with closely aligned, radially intermitting permanent magnets. In such instances the wall between the driving and driven magnets in liquid mixing applications is typically cooled by exposing the wall to the process liquid being pumped or mixed, or by exposing the wall to a coolant such as a cooling oil bath.
The application of a magnetically coupled agitator to mix dry or moist powders inside of a processing vessel is heretofore unknown in the prior art. More particularly, a method of using a magnetically coupled agitator to mix dry or moist powders inside of a rotating powder processing vessel operating under greater than or less than atmospheric pressure is heretofore unknown in the prior art.
In accordance with the method and device of the present invention, a magnetically coupled agitator is used for mixing dry or moist powders inside of a rotating tumble blender or other processing vessel. The vessel can be operated under a pressure greater than, equal to, or less than atmospheric pressure. The present invention eliminates the need for lip seals, mechanical seals and/or packing in the vessel walls, such as are used with agitators in the method of mixing powders in the prior art. Thus, in the present invention a conduit for cross-contamination between the powders being mixed and the environment outside of the mixing vessel is eliminated as is the possibility of cross-contamination between batches of different, sequentially mixed powders due to the entrainment of powders in the mechanical seals and/or packing. The present invention further eliminates the need for venting the processing vessel to the atmosphere during operation, thus limiting the likelihood of powders escaping from the processing vessel and reducing the threat to worker health or safety particularly where the powders being mixed are toxic or reactive.
Thus, it is one of the objects of the present invention to provide a processing vessel for mixing dry or moist powders which rotates about its axis and which comprises a rotating, magnetically coupled agitator.
It is a further object of the present invention to provide a new method for the batch or continuous mixing of dry or moist powders inside of a vessel which rotates about its axis and comprises a rotating, magnetically coupled agitator.
It is a further object of the present invention to provide a new method for the batch or continuous mixing of dry or moist powders inside of a rotating vessel which comprises a rotating, magnetically coupled agitator, which can operate under a pressure greater than or less than atmospheric and which avoids the problem of cross-contamination normally encountered where agitator driving mechanisms must enter through walls of a mixing vessel.
It is a further object of the present invention to provide a rotating processing vessel for mixing dry or moist powders which comprises a rotating, magnetically coupled agitator, which operates under a pressure greater than or less than atmospheric and which avoids the problem of cross-contamination normally encountered where agitator driving mechanisms must enter through walls of the processing vessel.
It is a still further object of the present invention to provide a method for mixing dry or moist powders inside of a pressurized vessel which comprises a rotating, magnetically coupled agitator, and which reduces the threat to worker health and safety.
It is yet a still further object of the present invention to provide a processing vessel for mixing dry or moist powders which rotates about its axis, which comprises a magnetically coupled agitator and which reduces the threat to worker health and safety.
In accordance with the foregoing objects, a rotating processing vessel for mixing dry or moist powders which comprises a rotating, magnetically coupled agitator is disclosed.
In further accordance with the foregoing objects, a method of using a magnetically coupled agitator to mix dry or moist powders inside of a rotating powder processing vessel is disclosed.
Briefly, the above and further objects are realized in accordance with the present invention by providing a substantially airtight processing or mixing vessel. Such a vessel is constructed to withstand operating pressures both in excess of and less than atmospheric pressure and, in some instances, may be a vessel constructed in accordance with the American Society of Mechanical Engineers (xe2x80x9cASMExe2x80x9d) Boiler and Pressure Vessel Code. The vessel shell may be of single or multiple-walled construction and is made of a substantially non-magnetic material such as a substantially non-magnetic metal, alloy (such as stainless steel or Hastelloy(copyright)), plastic or other material. Vessel dimensions will vary from application to application but generally range from about one (1) foot in diameter and about one and one-half (1xc2xd) feet in height to about ten (10) feet in diameter and about fifteen (15) feet in height.
The vessel is comprised of a shell, a substantially airtight cover, a substantially airtight main valve, a magnetically coupled agitator, a means for rotating the vessel about its axis and a means for rotating the agitator. The axis about which the vessel is rotated is preferably 30 degrees from the horizontal, more preferably 15 degrees from horizontal, and most preferably the vessel is rotated about its substantially horizontal axis. The vessel may also comprise a pressure relief valve. The cover is located in the top wall of the vessel and is either completely removable from the vessel or is hinged thereto.
To begin a batch mixing process either the main valve or the cover is opened. Both the main valve and the cover are sufficiently sized to permit ready entry into the vessel of the powder or powders to be mixed. Such powders are non-magnetic and have a moisture content of from about zero to about 50%. The following describes the batch mixing process where the cover has been chosen by the operator to be opened. After opening the cover the opened vessel is filled with powder to a predetermined level depending upon the particular mixing application. Some amount of freeboard is maintained between the top surface of the loaded powder and the top wall or the cover of the mixing vessel. This freeboard space permits the powder to move freely and expand during the tumble mixing and agitating process. Upon filling the vessel to the desired level with the powder to be mixed, the cover is closed and sealed so that the processing vessel is substantially airtight. The main valve is located in the bottom wall of the vessel and remains closed and substantially airtight during the powder loading operation.
The processing vessel being employed for the particular mixing application, such as a tumble blender, tumble dryer or other vessel suitable for mixing powders, is then rotated (tumbled) around its axis by means of a motor or other power source located externally from the vessel. The vessel is rotated in this manner at a speed of about two (2) to about thirty (30) rotations per minute. The time necessary to effect complete mixing, or drying if the vessel is a tumble dryer, varies from application to application but typically requires from about five (5) to about sixty (60) minutes for a mixing application and about one (1) to about one hundred (100) hours in a drying application.
To enhance or accelerate the tumble mixing process, or where the processing vessel is a tumble dryer to de-lump any lumped powder, an agitator is rotated within the rotating vessel by means of a magnetic coupling between the agitator and a second motor or other power source located externally from the vessel. The magnetic coupling comprises, in one embodiment, a driving member fixedly attached to a shaft which is rotatably driven by the power source. The driving member comprises a circular arrangement of a plurality of magnets. The driving member is concentrically received within a pocket provided by a trunnion or drive housing projecting internally into the interior of the processing vessel. The drive housing is formed in a manner to be continuous with the wall of the vessel. The drive housing is further configured for close reception of the driving member within the pocket without physical connection therewith.
The driven member is comprised of a circular arrangement of a plurality of magnets. The driven member is fixedly attached to a shaft which itself is attached to a plurality of agitator blades. The circular arrangement of magnets comprising the driven member is, in one embodiment, of a larger diameter than the circular arrangement of magnets comprising the driving member. The driven member is concentrically received over the inwardly projecting drive housing of the vessel and thereby also concentrically over the driving member. The driven member is configured for close reception of the drive housing without physical connection therewith. The driven member is coupled to a shaft for transmitting rotational movement to an agitator and a plurality of agitator blades for enhancing the mixing of the powder or powders contained within the vessel.
In operation, the driving and driven members are disposed for magnetic coupling with each other whereby, upon rotating the driving member, the driven member correspondingly rotates. That is, rotation of the drive shaft and the driving member secured thereto tends to distort the lines of force passing from the driving member to the driven member and the driven member is thereby forced to follow the rotation of the driving member. The strength of the magnetic linkage of the driving and driven members is directly related to the density of the magnetic flux passing between them. In that regard, the drive housing must be constructed of a substantially non-magnetic material in order to permit the magnetic field between the driving member and the driven member to permeate therethrough. If necessary, the driving member is cooled and lubricated by a cooling bath inside the drive housing using mechanical seals, a small pump, a reservoir and a small heat exchanger.
The agitator blades rotate at a tip speed of about 1650 feet per minute to about 5000 feet per minute. Because of the work energy necessary to rotate the agitator and the agitator blades fixedly attached thereto through the powder within the mixing vessel, heat energy is created within the vessel. The heat energy can, in some mixing applications, increase the temperature of the air inside of the vessel. Temperatures inside of the vessel during agitator mixing operations can increase up to about 80xc2x0 F. above the starting temperature in the vessel. In those mixing applications where a temperature rise occurs within the vessel, the air within the vessel expands, thus creating pressures in excess of the initial pressure inside of the vessel. Such pressure build-up inside the vessel during mixing of the powder with an agitator ranges from about two (2) pounds per square inch to about ten (10) pounds per square inch. Advantageously, this increase in pressure does not create the problems heretofore described. This is because in the present invention mechanical seals and/or packing have been eliminated from the vessel walls by using a magnetically coupled agitator. Thus, there is no possibility of failure of such seals and packing and hence there is no conduit for powders within the vessel to escape or become contaminated by the environment outside of the vessel. Similarly, because the need for atmospheric vents in the vessel have been eliminated by the present invention, there is no pathway for powders to escape from the vessel along this route. Additionally, because the need for mechanical seals and/or packing has been eliminated, the problem of batch to batch cross-contamination has also been resolved.
Once the powders within the vessel have been adequately mixed as determined by the time of mixing, the number of vessel rotations, the temperature within the vessel, or some other means, the power source for the agitator and then the power source for the rotating vessel are turned off, thus stopping the rotation of the agitator and the rotation of the vessel about its axis. Once the rotation of the agitator and vessel have halted, the vessel is oriented such that the main valve is at or near the bottom of the vessel and the cover is at or near the top of the vessel. Any build-up of pressure above the initial pressure within the vessel is then relieved either by opening the cover or by bleeding off the pressure prior to opening the cover by means of a relief valve attached to the cover or vessel wall. In either instance the pressure is exhausted to a contained dust collector system. If the pressure in the vessel is below atmospheric pressure, the pressure inside the vessel is equilibrated to atmospheric pressure by means of a process acceptable air source. This air source may be sterile.
Once any pressure within the vessel has been relieved, equalized and returned to atmospheric pressure levels, the main valve is opened and the mixed powders within the vessel are permitted to exit the vessel through the opened main valve to a subsequent processing step or into a container.
In the instance where the vessel operator chooses to load the powder into the processing vessel through the main valve instead of the cover, a mixing procedure similar to that hereinbefore described is followed except that the powder is loaded and then discharged, after mixing is complete, through the main valve.
It is to be understood that, in addition to the batch mixing process hereinbefore described, the mixing process may be also be operated in a continuous manner. That is, the powder or powders to be mixed may continuously be introduced into the rotating processing vessel while the magnetically coupled agitator is rotating, and the mixed powder or powders are also continuously withdrawn from the rotating processing vessel while the magnetically coupled agitator is rotating.
Further objects and advantages of the device and method of the present invention will be readily apparent to those skilled in the art and a better understanding of the present invention may be had by reference to the following detailed description taken in connection with the following drawings.